Closed loop color control in web offset lithography is most commonly accomplished by detecting optical density of color bars, which are arrays of solid patches on a paper substrate printed outside of the desired printed image. Each color bar is a continuous color target reference which is printed outside of the desired printed image and is present for every print zone in circumferentially the same position. Sensors measure the color bar and a controller adjusts the flow of ink in each ink zone of the inking unit based on the measured values of the color bar and target values. The flow ink into the ink zones is adjusted by adjusting the opening amount of ink keys in open fountain inking units or by adjusting the pulse rates of ink valves in digital ink rail inking units. U.S. Patent Publication 2007/0151470, which is hereby incorporated by reference herein, describes an open fountain inking unit that includes a plurality of ink keys (i.e., “screws”) and U.S. Pat. No. 7,171,900, which is hereby incorporated by reference herein, describes a digital ink rail inking unit that includes a plurality of ink valves.
U.S. Patent Publication 2007/0125246 discloses a complex method for measuring and controlling the color value of one or more colored image portions which are printed on a planar substrate in a plurality of ink zones that extend across a width of the substrate. The method includes dividing pixellated digital data into a plurality of digital paths corresponding to each of said ink zones, each digital path comprising a plurality of digital zones, and further dividing the pixellated digital data into color layers. The method also includes analyzing each of the color layers within each of the digital paths to determine a maximum pixel population area for each color within each of said digital paths.